Evolution of particle size distribution in suspension polymerisation reactions
Suspension polymerisation processes are commercially important for the production of polymer beads having wide applications. Polymers produced by suspension polymerisation can be directly used for particular applications such as chromatographic separations and ion-exchange resins. Particle Size Distribution (PSD) may appreciably influence the performance of the final product. Therefore, the evolution of PSD is a major concern in the design of a suspension polymerisation process. In this research, methyl methacrylate (MMA) has been used as a model monomer. A comparative study of MMA suspension polymerisation and MMNwater dispersion was carried out, for the first time, to elaborate the evolution of mean particle size and distribution. Polyvinyl alcohol (PVA) and Lauroyl Peroxide (LPO) have been used as stabiliser and initiator, respectively. Polymerisation experiments were carried out using a 1-litre jacketed glass reactor equipped with a turbine impeller and a condenser. The stabiliser, initiator and chain transfer concentrations, inhibitor concentration and type, reaction temperature, impeller speed, and monomer hold up were used as variables. A mathematical model was developed to predict the kinetics of polymerisation as well as the evolution of PSD by population balance modelling. The experimental results were compared with the model predictions. From the comprehensive experimental results, the characteristic intervals of a typical suspension polymerisation were realised as: 1) Transition stage during which PSD narrows dramatically and drop size decreases exponentially due to higher rate of drop break up in comparison with drop coalescence . _ until a steady state is reached. The importance, and even the existence, of the transition stage have been totally ignored in the literature. The results indicate that increasing the impeller speed, and PV A concentration will lead to a shorter transition period. Also increasing the rate of reaction, via increasing initiator concentration, and reaction temperature will shorten this period. ABSTRACT 2) Quasi steady-state stage during which the rate of drop break up and drop coalescence are almost balanced leading to a steady-state drop size and distribution. The occurrence of this stage is conditional. Low impeller speed and PV A concentration may remove the quasi steady-state stage completely and drops may start growing considerably after a sharp decrease in size during the transition stage. 3) Growth stage during which the rate of drop break up considerably falls below the rate of drop coalescence due to the viscosity build up in drops leading to drop enlargement and PSD broadening. Results show that the onset of the growth stage may not be fixed and it depends on the balance of the forces acting on drops. The onset of the growth stage in terms of time was advanced with decreasing stirring speed and PV A concentration and increasing monomer hold up. Under a static steady state, which is formed when a high concentration of PV A is used, there is almost no growth. 4) Identification stage during which a solid-liquid suspension is attained and the PSD and mean particle size remain unchanged afterwards. The onset of this stage appears to be fairly constant for different formulations. The developed model could fairly predict the rate of polymerisation. It was also capable of predicting the evolution of particle size average and distribution qualitatively in the course of polymerisation. The results can be used as a guideline for the control of particle size and distribution in suspension polymerisation reactors. A more quantitative exploitation of the model has been left for a future research.